Kept killing my boat battery every winter, then a guy at the marina in Sandusky showed me the $12 fix
For years I stored my 17 foot runabout in a rented garage over in Sandusky and every spring the battery was dead as a rock. I tried everything. Pulled it out and set it on a wood shelf in my basement, still dead. Trickle charger hooked up all winter, came back to a cooked battery and a $90 replacement. Then a guy two slips down asked why I was even leaving the battery in the cold. He said pull it, charge it full, then stick it on a piece of foam board on a shelf, not concrete. Recharge once a month, that is it. I did that in October and fired the motor up first turn in April. Cost me a $12 float charger from the hardware store and a scrap of foam I already had. Anyone else got a storage routine that sounds too simple to work but does?
The concrete thing is mostly a myth, modern batteries don't drain through the shelf, but the cold is what really kills them. Keeping it inside and topping it off once a month is the part that actually saved your battery.